The invention relates to a method for setting up an inbound call to a mobile telephone in a cellular mobile telephone network comprising a home location register for permanently storing location and subscriber data on mobile telephones registered with the network, the geographic coverage area of the network being divided into smaller location areas each comprising a mobile switching centre and a visitor location register for temporarily storing subscriber data on mobile telephones currently located within said location area, the geographic coverage area of each location area being further divided into radio cells each comprising at least one fixed radio station capable of establishing a radio link with the active mobile telephones currently located in the radio cell, in which method a first mobile switching centre receives the subscriber number of a mobile phone and requests the home location register to provide call routing information, and the home location register in turn requests a roaming number from the visitor location register within the location area of which the mobile telephone is currently located, and forwards the received roaming number to the first mobile switching centre for routing the call to the switching centre of the current location area and further to the desired mobile telephone.
Various cellular radio or mobile telephone systems are presently in use or being developed in which the geographic coverage area of the system is divided into smaller separate radio areas or cells in such a way that when the radio or mobile telephone is in a cell, it communicates with a fixed network through a fixed radio station located in the cell. Mobile telephones belonging to the system are free to travel from one cell to another within the area of the system. One such system is the digital mobile telephone system GSM (Groupe Special Mobile). When a subscriber within the same system or within an external system wishes to call a mobile subscriber (Mobile Station=MS) within this system, the fixed network must have information on the actual location of the mobile telephone MS in order to be able to route the call to the appropriate mobile services switching centre MSC. In the GSM system, for example, the number dialed by a calling subscriber contains no information on the actual location of the called MS. Therefore, to set up a complete connection, it is necessary to determine the location of the MS and the routing address, i.e. the mobile station roaming number, used for this purpose. In the GSM system, this information can be provided solely by the home location register (=HLR), which is a database in which subscriber data, such as location data, are permanently stored. Therefore, in order for it to be possible to route the call to the MSC within the area of which the MS is currently located, it is necessary to institute a query to the HLR.
In the GSM system, each location area consisting of a plurality of radio cells comprises a visitor location register VLR, which is a database in which a subscriber as well as data on the subscriber are stored while the subscriber visits the area of the VLR. The VLR provides the HLR with the information required for routing calls to the MS and participates in the switching of calls in the MSC, for instance.
There are two alternative modes of giving the routing information, i.e. a routing number. In the first mode, the roaming number can be provided when the VLR updates the location of the subscriber, and the roaming number can be stored in the HLR. In this case, the HLR returns the stored roaming number without further procedures when it receives a routing information query.
In the alternative mode, the HLR may request the VLR within the area of which the MS is currently located to provide a roaming number assigned exclusively to this call, when requested by a gateway MSC or one of the MSCs. The HLR forwards the roaming number provided by the VLR to the requesting MSC which routes the call to the appropriate MSC on the basis of the roaming number. After the call has been routed up to the MSC, the MSC initiates a mobile telephone paging procedure in its location area to find the radio cell and the fixed radio station within the area of which the MS is located. After having found the MS, the MSC establishes a radio link with the MS by means of this fixed radio station and sets up a speech connection.
If the VLR contains data indicating that the pertinent MS is currently inactive (for instance subscriber terminal turned off), the VLR forwards said information to the HLR. If the MS subscriber has activated a call transfer service to another telephone number, the HLR sends this call transfer number to the requesting MSC. The MSC thereafter institutes a query to the HLR whose subscriber has the call transfer number.